An electric double layer capacitor has a relatively large capacity and a long life-time, and allows a quick charge and discharge. Therefore, it has been used not only in conventional applications, such as a leveling of an electric power source or an absorption of noise, but also in a memory-backup power source for a personal computer, or as an auxiliary or substitutive source of a secondary battery. Recently, the electric double layer capacitor is expected to be used as a secondary battery for an electric vehicle.
The electric double layer capacitor has a structure wherein a pair of electrodes is immersed in an ionic solution. When a voltage is applied to the electric double layer capacitor, ions having a charge opposite to that of each electrode are distributed around each electrode to form layers of ions, whereas, in the electrodes, charges opposite to the outside ions are accumulated. Then, if a load is connected between the electrodes, the charges in the electrodes are discharged, and at the same time the ions distributed around the electrodes move away therefrom so that the capacitor is returned to a neutralized state.
If a pair of the electrodes is brought into contact with each other in the electric double layer capacitor, it becomes difficult to form the ion layer around the electrodes. Therefore, a separator is usually placed between the electrodes. The separator is required to have a property of avoiding a short circuit between the electrodes as above, and further a good property of holding an electrolyte and an ionic permeability.
If a thin separator is used as a separator for the electric double layer capacitor, the electric double layer capacitor can be thinned. Thus, porous membranes were proposed as the separator. For example, a separator made of a polyimide porous film having nonlinear fine holes was proposed (patent reference 1; particularly, in claim 1).
As other separators for the electric double layer capacitor, proposed were a separator of a nonwoven fabric prepared by a melt blowing method (patent reference 2; particularly, page 3, paragraphs [0046] to [0050]), a separator of a nonwoven fabric prepared by a wet-laying method using ultrafine fibers of island components obtained by removing a sea component in islands-in-sea fibers (patent reference 3; particularly, Examples), and a separator of a nonwoven fabric prepared by a wet-laying method using fibrillated organic polymer (patent reference 4; particularly, Examples).    [patent reference 1] Japanese Unexamined Patent Publication (Kokai) No. 2003-229329    [patent reference 2] US2002/0045091A1    [patent reference 3] Japanese Unexamined Patent Publication (Kokai) No. 2004-115980    [patent reference 4] Japanese Unexamined Patent Publication (Kokai) No. 2003-168629